The present invention relates generally to a system and method for actuating one or more valves in an internal combustion engine. In particular, the present invention relates to a system and method that may provide lost motion valve actuation of intake, exhaust, and auxiliary valves in an internal combustion engine.
Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, as well as to produce engine braking. During positive power, one or more intake valves may be opened to admit fuel and air into a cylinder for combustion. One or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves may also be opened during positive power at various times to recirculate gases for improved emissions.
Engine valve actuation also may be used to produce engine braking and exhaust gas recirculation when the engine is not being used to produce positive power. During engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle.
Engine valve(s) may be actuated to produce compression-release braking and/or bleeder braking. The operation of a compression-release type engine brake, or retarder, is well known. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. During engine braking operation, as the piston approaches the top dead center (TDC), at least one exhaust valve is opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of the Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is incorporated herein by reference.
The operation of a bleeder type engine brake has also long been known. During engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open continuously throughout the remaining engine cycle (full-cycle bleeder brake) or during a portion of the cycle (partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the former does not have exhaust valve lift during most of the intake stroke.
In many internal combustion engines, the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. Benefits such as increased performance, improved fuel economy, lower emissions, and better vehicle drivability may be obtained if the intake and exhaust valve timing and lift can be varied. The use of fixed profile cams, however, can make it difficult to adjust the timings and/or amounts of engine valve lift to optimize them for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to provide valve actuation that incorporates a xe2x80x9clost motionxe2x80x9d system in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, and/or other linkage assembly. In a lost motion system, a cam lobe may provide the xe2x80x9cmaximumxe2x80x9d (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve(s), and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application and which are incorporated herein by reference.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid valve and/or a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit when the master piston is acted on by certain of the cam lobes. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the hydraulic fluid displaced by the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the cam lobe acting on it. When the solenoid is opened, the circuit may drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston, and correspondingly, the engine valve.
Some previous lost motion systems have utilized high speed mechanisms to rapidly vary the length of the lost motion system. By using a high speed mechanism to vary the length of the lost motion system, precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions. Systems utilizing high speed control mechanisms, however, can be costly to manufacture and operate.
When a unitary cam lobe is used to impart the valve motion for both an auxiliary valve event (e.g., engine braking) and the main valve event (e.g., main exhaust), there may be increased overlap between the main intake and exhaust events. The use of a unitary lobe for both events means that the relatively large main event lobe motion will be imparted to the valve actuation system. Because there may be little or no lash between the valve actuation system and the engine valve during engine braking, input of the main event motion may produce a greater than desired main exhaust event. The time during the cycle when both intake and exhaust valves are open at the same time may be increased. The longer that both the intake and exhaust valves are open together, the more exhaust manifold pressure is likely to bleed through the open intake valve. This may greatly reduce braking performance. As such, there is often a need for a valve actuation system including a xe2x80x9cresetxe2x80x9d mechanism, such that, when a unitary cam lobe is used to impart the valve motion, the valve experiences normal lift and closing during engine braking.
The design, size, and configuration of many engines require valve actuation systems to be located relatively remote from the engine valves that they are required to actuate (e.g., on the input side of an engine rocker arm), rather than being located on the valve side of the engine. Production tolerances for components on the input side of an engine rocker arm (e.g., the push tube) are typically much greater than those on the valve side because the manufacturer may anticipate making manual lash adjustments. Incorporating valve actuation systems capable of providing precise lost motion and/or reset functionality in this location may be difficult due to the inherent production tolerances that may exist between the valve actuation system and the valves.
The lost motion systems and methods of the present invention may be particularly useful in engines requiring lost motion valve actuation for positive power, engine braking valve events (such as, for example, compression release and bleeder braking), and/or exhaust gas recirculation valve events. The systems of various embodiments of the present invention may provide a lower cost, production viable lost motion circuit with fixed event timing that requires no high speed electronic controls to operate. In addition, the systems and methods of the present invention may reduce valve overlap during braking and reduce the impact on the valve train.
Additional advantages of embodiments of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.
Responsive to the foregoing challenges, Applicant has developed innovative systems and methods for actuating one or more engine valves. In one embodiment, the present invention is an engine valve actuation system comprising: a lost motion subsystem operatively connected to the engine valve; a hydraulic fluid supply in communication with the lost motion subsystem; and means for imparting motion to the lost motion subsystem. The lost motion subsystem may comprise: a housing having an internal bore; a piston assembly slidably disposed in the bore, the piston assembly comprising a master piston and a slave piston; a hydraulic control valve; a solenoid actuated hydraulic fluid valve; a first hydraulic passage connecting the control valve to the piston assembly; a second hydraulic passage connecting the fluid supply to the control valve; and a third hydraulic passage connecting the solenoid valve to the control valve.
In another embodiment, the present invention is a method of actuating an engine valve during first and second operating modes to produce a main event valve actuation and to selectively produce an auxiliary event valve actuation using motion imparted to a lost motion subsystem. The method may comprise the steps of: supplying hydraulic pressure to the lost motion subsystem; during the first operating mode, selectively absorbing at least a portion of the hydraulic pressure applied to the lost motion subsystem so as to selectively lose a portion of the motion imparted thereto; and during the second operating mode, creating a hydraulic lock in the lost motion subsystem to transfer the motion to the engine valve and selectively modifying the manner in which the motion is transferred to the valve from hydraulic means to mechanical means.